How to transform a mesh with many objects?
Hook and Vertex Parent.

Triangle in edit mode.



Select each vert, hit CtrlH and choose add new hook.  This creates an empty at the vert.

Repeat for other two verts.

For each empty, 



Select empty, Shift select cube to parent to.
Go into edit mode, select one, two or three vertices then CtrlP To make vertex parent.

Note could cut out the "middle man" empty and hook directly to cubes.